(1) Field of the Invention
The present invention relates to a process for treating surfaces of substrates coated with a chemical in a solvent using optical energy to produce surfaces with the chemical bonded to it. In particular, the present invention relates to a preferred process for pretreating surfaces of substrates by providing water containing the chemical between the optical energy source and the substrate.
(2) Description of the Related Art
Manufactured surfaces of substrates always contain undesirable compounds or additives that limit or reduce adhesion to an adhesive or paint film. Hence, surface preparation, which includes cleaning and activation of the surfaces, of polymeric, polymer composite or metal substrates is carried out prior to applying protective paint films or adhesive bonding. Surface preparation determines the mechanical and durability characteristics of the composite created. Currently the techniques used for surface preparation are mechanical surface treatments (e.g. abrasion), solvent wash and chemical modification techniques like corona, plasma, flame treatment and acid etching. Each of the existing processes have shortcomings and thus, they are of limited use. Abrasion techniques are found to be time consuming, labor intensive and have the potential to damage the adherent surface. Use of organic solvents results in volatile organic chemical (VOC) emissions. Chemical techniques are costly and are of limited use with regard to treating three dimensional parts, can be a batch process (such as plasma, acid etching) and need tight control.
The use of lasers for surface treatment is known in the art. The focussed beams of the lasers make it difficult to treat a large surface. U.S. Pat. No. 4,803,021 to Werth et al. describes such a method. U.S. Pat. No. 4,756,765 to Woodroffe describes paint removal with surface treatment using a laser.
Plasma treatment of surfaces is known in the art. Relatively expensive equipment is necessary for such treatments and plasmas are difficult to control. The surfaces are treated with vaporized water in the plasma. Illustrative of this art are U.S. Pat. No. 4,717,516 to Isaka et al., U.S. Pat. No. 5,019,210 to Chou et al., and U.S. Pat. No. 5,357,005 to Buchwalter et al.
A light based process which cleans a substrate surface also creates a beneficial chemistry on the surface for adhesive bonding and paintability is described in U.S. Pat. No. 5,512,123 to Cates et al. The process involves exposing the desired substrate surface to be treated to flashlamp radiation having a wavelength of 160 to 5000 nanometers. Ozone is used with the light to increase the wettability of the surface of the substrate being treated. Surfaces of substrates such as metals, polymers, polymer composites are cleaned by exposure to the flashlamp radiation. The problem with the Cates et al. process is that the surface of the substrate is heated to a relatively high temperature, particularly by radiation above 500 nanometers and relatively long treatment times. Related patents to Cates et al. are U.S. Pat. No. 3,890,176 to Bolon, U.S. Pat. No. 4,810,434 to Caines; U.S. Pat. No. 4,867,796 to Asmus et al.; U.S. Pat. No. 5,281,798 to Hamm et al. and U.S. Pat. No. 5,500,459 to Hagemeyer et al. and U.K. Patent No. 723,631 to British Cellophane. Non-patent references are: Bolon et al., “Ultraviolet Depolymerization of Photoresist Polymers”, Polymer Engineering and Science, Vol. 12 pages 109–111 (1972). M. J. Walzak et al., “UV and Ozone Treatment of Polypropylene and poly(ethylene terephthalate)”, In: Polymer Surface Modification: Relevance to Adhesion, K. L. Mittal (Editor), 253–272 (1995); M. Strobel et al., “A Comparison of gas-phase methods of modifying polymer surfaces”, Journal of Adhesion Science and Technology, 365–383 (1995); N. Dontula et al., “A study of polymer surface modification using ultraviolet radiation”, Proceedings of 20th Annual Adhesion Society Meeting, Hilton Head, S.C. (1997); C. L. Weitzsacker et al., “Utilizing X-ray photoelectron spectroscopy to investigate modified polymer surfaces”, Proceedings of 20th Annual Adhesion Society Meeting, Hilton Head, S.C. (1997); N. Dontula et al., “Ultraviolet light as an adhesive bonding surface pretreatment for polymers and polymer composites”, Proceedings of ACCE'97, Detroit, Mich.; C. L. Weitzsacker et al., “Surface pretreatment of plastics and polymer composites using ultraviolet light”, Proceedings of ACT'97, Detroit, Mich.; N. Dontula et al., “Surface activation of polymers using ultraviolet activation”, Proceedings of Society of Plastics Engineers ANTEC'97, Toronto, Canada. Haack, L. P., et al., 22nd Adhesion Soc. Meeting (Feb. 22–24, 1999).
Non-pulsed UV lamps have been used by the prior art. These are described in: “Experimental Methods in Photochemistry”, Chapter 7, pages 686–705 (1982). U.S. Pat. No. 5,098,618 to Zelez is illustrative of the use of these types of lamps.
A disadvantage of the ultraviolet lamp treatments of the prior art is that they are time consuming and sometimes unreliable. To achieve suitable surface chemistries for adhesive bonding and painting purposes, exposure times for certain materials like polypropylene, thermoplastic olefins (TPO's) tend to be of the order of 5 to 60 minutes. In many cases there is a limit on the length of time to which one may expose the substrates to UV since there is a fear of degrading the substrate. There is a need for development of an environmentally friendly as well as cost effective and robust surface treatment process which can be used over a range of surfaces.